1. Field of the Invention
This invention pertains generally to measurement of analyte in gas-borne particles, and particularly to collection of gas-borne particles into a liquid so that analytes in the particles can be analyzed.
2. Description of the Related Art
Devices are needed that can quickly detect and identify the presence of materials, such as harmful materials, in airborne particles. Airborne infectious agents such as bacteria and viruses can transmit diseases of humans, other animals, and plants. Some of these infectious agents, as well as some protein toxins, have been used as biological-warfare (BW) agents. Some airborne proteins and pollens cause allergies. Here, “airborne particles” or “gas-borne particles” refer to both the solid particles and liquid droplets in an air or gas sample.
A group of particles in an air sample, of either indoor or outdoor air, may include many types of organic and inorganic materials and many types of bacterial and other biological materials. Individual particles may be homogenous or may be a complex mixture of materials. Identification of specific bacteria, viruses, or proteins, and many other types of complex chemicals that may be mixed in atmospheric particles requires the collection of those particles so that they can be analyzed by appropriate techniques, e.g., immunoassay for bacteria, protein allergens, and other biological materials; culturing for bacteria and viruses; mass spectrometry for organic chemicals. For identification of specific bacteria or proteins using biochemical methods, the collection of these particles into a liquid is needed. For many types of analyses, e.g., single-particle infrared or Raman analysis, collection of airborne particles onto filters or other solid surfaces is desired.
An overview of aerosols and some of the principles relevant for collection of aerosols are provided by William C. Hinds, Aerosol Technology, Properties, Behavior, and Measurement of Airborne Particles (Wiley, New York, 1999) 2nd edition, which is herein incorporated by reference. Throughout this application we will refer to this as “Hinds” and may give specific pages, equation numbers, etc.
Investigators have worked for years to develop instruments for collecting aerosol particles into liquids. The literature describing different methods is large. Methods for collection of airborne particles, including methods for collection of particles into liquids, have been reviewed by M. L. Muilenberg, “Sampling Devices,” Immunology and Allergy Clinics of North America, 23, 337-355 (2003). Liquid-impingement methods are widely used, but the volumes of liquid needed for microfluidic analyses are typically far smaller than are used with liquid impingers.
Some methods of collecting particles into a liquid are briefly summarized in the paper by I. Belostotski, V. V. Gridin, I. Schecter, and C. N. Yarnitzky, “Combined micro-droplet and thin-film-assisted pre-concentration of lead traces for on-line monitoring using anodic stripping voltammetry,” Analytical Bioanalytical Chemistry, 375, 482-488 (2003), especially in the introduction of that paper.
S. Zaromb, “High-Throughput Liquid-Absorption Air-Sampling Apparatus and Methods,” U.S. Pat. No. 6,087,183, describes an apparatus to collect particles from air into liquid. In his apparatus, 200 to 300 liters/min of air are drawing asymmetrically into a cylinder. This rapid flow of air swirling in the cylinder causes the liquid at the bottom of the cylinder to rise and also swirl inside the cylinder. Then particles from the air are collected into the liquid. The apparatus uses at least a few milliliters of liquid per sample, and evaporation may be expected to be significant.
P. T. Call, V. M. Kenning, C. Call, J. G. Birmingham, and D. J. Hammerstrom, “Impact Particulate Collector Using a Rotary Impeller for Collecting Particulates and Moving a Fluid,” U.S. Pat. No. 6,267,016 B1 (2001), describe another means to collect particles into a liquid. The airborne particles impact upon a rotating surface. A liquid is injected into the cavity that contains this rotating surface, and the particles may be washed into this liquid or may impact directly into the liquid.
P. J. Coyle, T. A. Pletcher, T. J. Davis, and S. Mangru, “Method and Apparatus for Concentrated Airborne Particle Collection,” U.S. Patent Application Publication, 2004/006907 A1, which is herein incorporated by reference, describe an apparatus to collect particles from a large volume of air into a relatively small volume of liquid in which a hydrophobic membrane establishes a “controllable air-fluid boundary” between a separation section where the particles are in air, and a capture section in which the particles are transported in liquid. A corona charger may be used to charge the aerosol so that the particles may be focused into the liquid stream.
Agranovski, V. Agranovski, S. A. Grinsphun, T. Reponen, and K. Willeke, “Collection of Airborne Microorganisms into Liquid by Bubbling through Porous Medium,” Aerosol Science and Technology, 36, 502-509 (2002), describe a means to collect aerosols by bubbling them through a fibrous filter that is immersed in water. They demonstrate very efficient collection for an air sample of up to 4 liters/minute running for up to 8 hours.
D. A. Masquelier, F. P. Milanovich and K. Willeke, “High Air Volume to Low Liquid Volume Aerosol Collector,” U.S. Pat. No. 6,520,034 B1, which is herein incorporated by reference, provide an apparatus to efficiently collect particles from a high volume (e.g., 225 Lpm) of air into a “collecting liquid having a volume not greater than about 100 to 300 microliters.” They provide a means to reduce the loss of the collection liquid, which they need to reduce because they impinge 225 liters/min of air onto this liquid in order for the particles in the air to be transferred to the liquid. This impinging of air into the liquid causes some of the liquid to form airborne droplets and some of the liquid to evaporate. They collect for re-use the liquid droplets, but not the evaporated liquid. The device is, small (typically 2 liters), lightweight (typically less than 1 Kg), and has low power requirements.
V. V. Gridin, I. Litani-Barzilai, M. Kadosh, and I. Schecter, “A Renewable Liquid Droplet Method for On-Line Pollution Analysis by Multi-Photon Ionization,” Analytical Chemistry, 69, 2098-2102 (1997), which is herein incorporated by reference, describe collection of gasses and aerosol particles into a pendant water droplet, where they are analyzed. The collection efficiency is not expected to be large, but would be sufficient for cases where particles that are in high concentration, and/or when the collection times are long.
V. Gartstein and A. D. Willey, “Dynamic Electrostatic Aerosol Collection Apparatus for Collection and Sampling Airborne Particulate Matter,” U.S. Patent Application Publication, 2004/0089156 A1, published May 13, 2004, describe a method of collection of aerosol particles by passing them through a spray of charged droplets that act as electrostatic collectors. The particles may be given a charge opposite to that of the droplets to enhance the collection efficiency. The liquid is re-circulated and is chosen to have a low volatility to reduce evaporative losses. See also J. B. Fenn's, “Electrospray Air Sampler,” U.S. Patent Application Publication 2004/0023411 A1, published Feb. 5, 2004.